McRoberts' maneuver is recommended as the initial technique for alleviating shoulder dystocia.1 The maneuver is simple to apply, involving hyperflexion of the woman's legs, and has been associated with trends towards lower rates of maternal and neonatal morbidity.1 When used alone, McRoberts' maneuver has alleviated approximately 40% of all shoulder dystocia cases. The rate of successful resolution of shoulder dystocia rises to nearly 54% when McRoberts' maneuver is combined with suprapubic pressure or proctoepisiotomy or both. Objective testing has also shown that McRoberts' maneuver can reduce fetal shoulder extraction forces and brachial plexus stretching.2
Little is known about how McRoberts' maneuver alleviates shoulder dystocia. The only information in the literature on its effects on pelvic contour is a single x-ray analysis by Gonik and Stringer.3 Using x-ray analysis, our objective was to document systematically the changes in pelvic dimensions created by McRoberts' maneuver.
Materials and Methods
All women who presented to the triage area of the labor and delivery unit were eligible for study entry. Consecutive women who met inclusion criteria were queried about study participation. The length of study enrollment was for the time necessary to take the x-rays. The study was approved by the investigational review board of Naval Medical Center Portsmouth. All subjects had completed at least 36 weeks' gestation, based on dating criteria of last menstrual period (LMP) or first-trimester ultrasound. All women were counseled regarding fetal and maternal risks of irradiation and those enrolled provided informed consent. No financial incentive was offered for participation. Exclusion criteria included active labor, imminent delivery of the fetus, nonreassuring fetal heart rate (FHR) tracing, hypertensive crisis of pregnancy, unexplained vaginal bleeding, evidence of placental abruption or placenta previa, multiple gestation, inability to position the woman due to pain of uterine contractions, degenerative changes in hips or spine, or severe sciatica.
According to the Colcher-Sussman technique,4 anterior-posterior and lateral pelvimetry was done with women in the dorsal lithotomy position. Similar x-rays were done after application of McRoberts' maneuver, in which maternal legs were hyperflexed 45 degrees onto the maternal abdomen.5 We compensated for radiographic magnification by including a metallic ruler for comparison. One of the investigators was present at all times during pelvimetry studies to assure proper application of McRoberts' maneuver. Obstetric providers were masked to results of pelvimetry.
All x-rays were read after delivery to ascertain6 anterior-posterior and transverse diameters of the pelvic inlet, mid-pelvis, and pelvic outlet; the true (anatomic), diagonal, and obstetric conjugates; the degree of symphyseal separation; and the distance between the symphysis pubis and the top of the fifth lumbar vertebra (Figure 1).
Based on the initial x-ray described by Gonik,3 we did an a priori power calculation. In his description of McRoberts' maneuver, a 61.5% decrease in the angle of inclination of the fifth lumbar vertebra was noted. To confirm that decrease, we determined that 64 subjects were needed to achieve 80% power, with alpha = .05.
Maternal charts were reviewed for age, gravidity, parity, estimated gestational age at study entry, height, weight, and presence of diabetes mellitus. From neonatal and maternal obstetric records we determined mode of delivery, birth weight, whether shoulder dystocia was present, and if brachial plexus injury had occurred. A two-tailed paired t test was used to determine the significance of the changes in the pelvic diameters after application of McRoberts' maneuver. P < .05 was considered statistically significant.
None of the 36 participants withdrew after giving informed consent. Table 1 presents demographic data, obstetric information, and selected neonatal characteristics. Twenty-nine women were enrolled in early labor, three x-rays were taken before elective repeat cesarean deliveries, and four were taken before external cephalic version. Among the 30 women who delivered vaginally, one had shoulder dystocia after delivery of a 4960 g infant who had no evidence of brachial plexus injury. We excluded from analysis two women in whom pelvic diameters could not be assessed due to inadequate penetration of x-rays or inability to view the sacrum.
As seen in Table 2, we noted no significant changes in anterior-posterior and transverse diameters of pelvic inlet, mid-pelvis, and pelvic outlet. The obstetric, true, and diagonal conjugates also did not increase when McRoberts' maneuver was applied. There was no significant increase in measured distance between the top of the symphysis pubis and the top of the fifth lumbar vertebra, the angle originally described in Gonik's case report.
When women were given McRoberts' maneuver there was a marked cephalad rotation of the symphysis pubis, evidenced by statistically significant increases in the angle of inclination between the top of the symphysis and the top of the sacral promontory, and a 24% decrease in the angle created by a line bisecting the symphysis relative to the horizontal (Table 3, Figures 2 and 3). We adequately calculated the angle created by a line bisecting the longitudinal axes of L5 and the upper sacrum in 30 (83.3%) of the x-rays. The sacrum flat-tened, with a decrease in the angle after McRoberts' maneuver (P = .04). In only one woman was the angle unchanged from the dorsal lithotomy position angle. All other women showed some degree of decrease in angle.
According to a literature search using MEDLINE, covering 1966 to February 1999, using the search terms “McRoberts,” “pelvimetry,” “x-ray,” “irradiation,” “pregnancy,” and “shoulder dystocia,” this study is the first series of systematic observations showing benefit in the maternal pelvis from McRoberts' maneuver. McRoberts' maneuver does not change the actual dimension of the maternal pelvis, it straightens the sacrum relative to the lumbar spine, with a cephalic rotation of the symphysis pubis sliding over the fetal shoulder.
Also, McRoberts' maneuver is believed to provide several additional benefits,5 which include allowing the posterior fetal shoulder to pass over the sacrum and through the pelvic inlet and the plane of the pelvic inlet to move perpendicular to the maximum maternal expulsive vector force.
The mechanism of action of McRoberts' maneuver has been little investigated since its brief description 15 years ago. In a single x-ray of McRoberts' maneuver, Gonik noted that the symphysis pubis rotated superiorly by 8 cm, freeing the impacted anterior shoulder.3 The same radiograph showed that the angle of inclination between the symphysis and the fifth lumbar vertebra was reduced from 26 to 10 degrees.
Williams7 examined 106 women with clinical pelvimetry and found the anterior-posterior diameter of the pelvic outlet increased by 1.75 cm with a change from the dorsal recumbent to the extreme lithotomy position. DeLee's classic 1913 textbook of obstetrics notes that “… any tendency to pendulous belly is corrected; the fetus is straightened out, the levator ani tightened (which facilitates anterior rotation of the occiput), and the outlet of the bony pelvis is enlarged” by the exaggerated lithotomy position.8 Walcher's position, a reverse form of McRoberts' maneuver in which the thighs are hyperextended, results in downward displacement of the symphysis pubis by 1–1.5 cm.9 In a radiographic study of 40 women during labor, the lithotomy position resulted in a 2–3 cm upward displacement of the symphysis pubis. When the fetal head presented in the pelvic outlet, the lithotomy position created a palpable upward displacement of the symphysis.10 In his discussion of delivery positions, Russell noted that “… if the thighs are flexed and abducted … by the mother who pulls hard her knees cranially in the second stage … the femora act as levers on the innominate bones to open the bony outlet.”11,12
Two recent reports suggested that overly aggressive hyperflexion of the maternal legs might be associated with symphyseal separation and transient femoral neuropathy,13,14 which implied the symphysis pubis represented a potential site of action of McRoberts' maneuver. But we found no significant increase in degree of symphyseal separation with application of McRoberts' maneuver.
After 36 women were enrolled, statistical analysis was done on our primary outcome measure, the angle of inclination of the fifth lumbar vertebra, which showed a power of 0.99 to detect a difference and an alpha of .04. The effect size of 3.393 indicated that a large difference was found, far larger than originally estimated when the sample was calculated. Those values supported prematurely terminating the study without enrolling the total subjects suggested by prestudy sample calculation.
Although we attempted to image the pelvis in the same way in all cases, variations in maternal body habitus and pelvic contours could have introduced study bias. Variations in focus-film and focus-target distances might not have been constant, producing errors in radiographic magnification. Also, maternal diabetes might have significantly reduced benefits of McRoberts' maneuver. We were unable to evaluate the effect of fetal size or presentation on changes created by McRoberts' maneuver, which might have been possible if we had used Ball's pelvimetry technique, which uses corrected values for diameters of pelvic planes to calculate spheres, which are compared with the calculated volume of fetal cranium.15
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13. Gherman RB, Ouzounian JG, Incerpi MH, Goodwin TM. Symphyseal separation and transient femoral neuropathy associated with the McRoberts' maneuver. Am J Obstet Gynecol 1998;178:609–10.
14. Heath T, Gherman RB. Symphyseal separation, sacro-iliac joint dislocation, and transient lateral femoral cutaneous neuropathy associated with McRoberts' maneuver. J Reprod Med. In press.
15. Friedman EA, Taylor MB. A modified normographic aid for x-ray cephalopelvimetry. Am J Obstet Gynecol 1969;105:1110–5.